1. Field of the Invention
The present invention relates to a starter having an epicyclic gear reduction unit for reducing the rotation of a motor output shaft integrated with an armature and transmitting the reduced rotation to a starting output shaft and, particularly, to an impact absorbing unit for a starter which absorbs impact stress caused by excessive rotation torque on a load side.
2. Description of the Prior Art
FIG. 14 is a partly exploded side view of a starter incorporating an epicyclic gear reduction unit of the prior art and FIG. 15 is a front view showing key parts of the mated internal gear of the epicyclic gear reduction unit in the starter of the prior art.
In FIG. 14 and FIG. 15, the starter comprises a starting motor 3 for generating rotation force, an epicyclic gear reduction unit for reducing the rotation of the motor output shaft 4 of this starting motor 3 and outputting the reduced rotation, an overrunning clutch 7 which is fitted onto the starting output shaft 6 of the epicyclic gear reduction unit 5, a pinion 8 which can slide over the starting output shaft 6 together with this overrunning clutch 7, and an electromagnetic switch 9 for controlling power supply to the starting motor 3 and urging the pinion 8 toward the ring gear 14 of an engine together with the overrunning clutch 7 through a shift lever 10.
This starting motor 3 is formed like a bottomed cylinder and comprises a yoke 11 which also functions as an outer frame and magnetic circuit, a field coil 12 placed on the yoke 11, an armature 13 placed on the inner side of the field coil 12, a rectifier (not shown) attached to the motor output shaft 4 which is the rotation shaft of the armature 13, and a brush (not shown) in sliding contact with the rectifier. A rear bracket 2 is mated with the rear end of the yoke 11 to be connected to the yoke 11 so as to supports the rear end of the motor rotation shaft 4. A front bracket 1 is mated with the front end of the yoke 11 to be connected to the yoke 11.
This epicyclic gear reduction unit 5 comprises a sun gear 15 formed around the front end of the motor rotation shaft 4, a plurality of epicyclic gears 16 which mesh with the sun gear 15 and an internal gear 17 which meshes with the epicyclic gears 16.
As shown in FIG. 15, the internal gear 17 is formed like a bottomed cylinder having a center hole in the center of the bottom portion, rotation stoppers 19 formed on the outer wall and an internal gear portion 18 formed on the inner wall. The internal gear 17 is fixed to the front bracket 1 so that it is open at the rear end (on the armature side). The rotation stoppers 19 are mated with the front bracket 1 to limit the movement in a circumferential direction of the internal gear 17. The rotation stoppers 19 are required to transmit torque required by the engine from the motor rotation shaft 4 to the starting output shaft 6.
A disk-like flange portion 20 is formed at the rear end of the starting output shaft 6 integratedly. A plurality of pins 21 are provided on the rear end surface of the flange portion 20 concentrically at an equal angular pitch. The epicyclic gears 16 are supported by the respective pins 21 rotably. This flange portion 20, namely, the starting output shaft 6 is rotably supported by a bearing 22 fixed in the center hole of the internal gear 17 fixed to the front bracket 1, and the front end portion of the motor output shaft 4 is rotably supported by a bearing 23 fixed to the flange portion 20. Thereby, the plurality of epicyclic gears 16 mesh with the sun gear 15 and the internal gear portion 18 to constitute the epicyclic gear reduction unit 5.
The sun gear 15 rotates together with the motor rotation shaft 4 to transmit the rotation of the motor rotation shaft 4 to each of the epicyclic gears 16. The epicyclic gears 16 revolve round the sun gear 15 by the rotation of the sun gear 15 while they rotate on their own axes. The starting output shaft 6 is rotated by this revolution of the epicyclic gears 16.
The overrunning clutch 7 can move in an axial direction and is fitted onto the starting output shaft 6. The pinion 8 is connected to the front end portion of a sleeve shaft 24 constituting the overrunning clutch 7. A shift lever 10 is installed such that it can rotate on an intermediate fulcrum portion 10a, one end of which is mated with the overrunning clutch 7 and the other end is connected to the plunger (not shown) of the electromagnetic switch 9 installed above the starting motor 3.
A description is subsequently given of the operation of this starter of the prior art constituted as described above.
Before the operation of the starter, the shift lever 10 is first located at a position shown in FIG. 14, the overrunning clutch 7 does not move, and the pinion 8 is not mated with the ring gear 14.
Then, when a key switch (not shown) is closed and the starter is activated, the armature 13 is energized with power supplied from the electromagnetic switch 9 and rotated by the energization force of the field coil 12. The motor output shaft 4 integrated with the armature 13 is rotated by the rotation of the armature 13. The shift lever 10 is driven by the plunger of the electromagnetic switch 9 and turned on the fulcrum portion 10a in the counterclockwise direction of FIG. 14. The overrunning clutch 7 is pressed by the rotation of this shift lever 10 and moves the starting output shaft 6 forward (right direction in FIG. 14) together with the pinion 8, and the pinion 8 is thereby mated with the ring gear 14.
At this point, rotation torque output from the armature 13 is transmitted from the sun gear 15 of the motor output shaft 4 to the epicyclic gears 16. The epicyclic gears 16 rotate on the respective pins 21 between the sun gear 15 and the internal gear portion 18. That is, the epicyclic gears 16 revolve round the sun gear 15 while they rotate on their own axes. The flange portion 20 supporting the epicyclic gears 16 rotates at a speed lower than the rotation speed of the motor output shaft 4 by the revolution of the epicyclic gears 16 and transmits this reduced rotation output to the starting output shaft 6. The starting output shaft 6 rotates the ring gear 14 (crank shaft) through the overrunning clutch 7 and the pinion 8 at a reduced speed.
In this type of starter, the crank shaft which is connected to the starting output shaft 6 to be rotated may stop suddenly, or the starting output shaft 6 being rotated may be connected to the crank shaft suddenly. In this case, excessive rotation torque is applied to the starting output shaft 6 all of a sudden. The rotation torque applied to the starting output shaft 6 is transmitted from the flange portion 20 at one end of the starting output shaft 6 to the epicyclic gears 16, then to the internal gear 17 through the internal gear portion 18 and finally to the motor output shaft 4 through the sun gear 15.
In the above starter of the prior art, as the internal gear 17 and the front bracket 1 are connected to each other by the rotation stoppers 19, impact stress caused by excessive rotation torque on a load side which undergoes a sudden change may be transmitted to the front bracket 1 and the motor output shaft 4 through the epicyclic gear reduction unit 5 and break a fragile portion of each element of the output transmission system of the starter.